Since the early 1960s, artificial heart valve prostheses have enjoyed tremendous success. Reliability and sustained high performance has been achieved by the use of very hard and exotic materials to form the structure. Such heart valve prostheses are exemplified in the following U.S. Pat. Nos. 3,534,411-Shiley; 3,812,542-Shiley; 3,824,629-Shiley; 4,057,857-Fettel, and the disclosures of these patents are incorporated herein by reference. Typically, such heart valve prostheses have a ring structure. Attached to the ring structure is usually some form of strut or pendant structure of elongated shape which supports the opening and closing of a discoid occluder. This occluder is disposed over the ring structure and substantially seals the orifice defined by the ring structure. In the art of heart valve prostheses, a constant search for improvement of the valve has continued since the early 1960s. The goals for such a search have been the reduction of thrombus formation in the heart within the vicinity of the heart valve through the reduction of stasis in blood flow (which stasis is roughly proportional to the size of the obstruction to blood flow presented by the heart), and concurrent with this goal, the goal of increasing the structural integrity of the heart valve. However, it has seemed that these two goals were incompatible in that increasing the structural integrity of the valve involved increasing the size of the structural members associated with the valve. Such an increase in size presents a greater obstruction presented to the blood flow with a concurrent increase in stasis, which thus increases the potential for thrombus formation. Thus, the search has continued for a method for improvement which would reconcile the two seemingly competing and irreconcilable goals of structural improvement and blood flow improvement.
Typically, in the prior art, the pendant structures or struts, attached to the valve ring, are formed of very hard metal wire, such as Haynes 25 material or a cobalt alloy, which is cut and welded to the ring structure after being shaped to the appropriate shape. Such welding necessarily results in the formation of a weld fillet extending beyond the welded juncture between the strut and the ring which has to be accommodated on the surface of the valve ring. Furthermore, these wire struts have a circular cross-section over which the blood has to flow. Although a circular cross-section for an obstruction sitting across the blood flow is not the most favorable cross-section for maximizing the flow across the valve, attempts to modify the circular cross-sectional configuration of the wire strut were generally unsuccessful because the machining of such a hard metal elongated wire strut leads to overheating of the wire and consequent distortion of the wire. These effects are due to the hard metal from which the wire must be formed and the prolonged machining time required to shape such a wire.
The prior art generally taught against integral formation of the strut and the valve ring in heart valves. The use of very hard metallic alloys, such as Haynes 25 material, or various alloys of cobalt, made the machining of such metal extremely difficult. The intricate shape of the strut necessary to permit the wobbly tilting action disclosed in the Fettel patent, U.S. Pat. No. 4,057,857, and the unique rocking occluder action also disclosed in that patent, made the machining of the strut even more difficult and seemingly impossible. The cross-sectional area of the strut is thin compared to its length, and this was a further obstruction in the machining of such a strut. It was found, for example, when the integral formation of the strut and ring was attempted through numerical control machining of a disc-shaped blank, the small size and intricate shape of the strut caused overheating and distortion in the strut because of the low heat conductive characteristics of the thin strut and because of the lengthy machining time necessary to achieve the intricate shape required for the strut. In stamping, it was found that the edge finish of the strut was not acceptable and the teardrop shape could not be sheared properly. When forging was attempted to form the integral strut and ring structure from a disc blank, the surface finish was bad as a result of the forging process and the costs in machining the finish after forging were prohibitive. Investment casting was also attempted in order to form the strut integral with the valve ring. However, such casting resulted in a porous surface finish which again resulted in the requirement for additional costly machining in order to obtain a good surface finish. As is well known in the art, a smooth surface finish is necessary in order for the heart valve prosthesis to work successfully. Also, the formation of voids within the structure by casting presented a serious risk. Laser machining was also attempted to form the strut and ring integral with one another from a disc blank. However, the teardrop shape of the strut could not be formed with laser machining and the inside ring radius could not be formed. Because of the tight dimensional tolerances inherent in the heart valve design, electrical discharge machining was found to be impractical because dimensional control in such machining is difficult due to erosion of metal from the electrode of the tool. Furthermore, the outer periphery of the ring could not be formed using electrical discharge machining. Finally, in the effort to form the strut and the valve ring to be integral with one another, chemical milling was attempted. However, because of the slow etch rate of the hard material from which the valve ring and strut must be formed, the mask used in the chemical milling was unavoidably undercut in the attempt, and the shape of the resulting structure was distorted. Thus, it seemed apparent in the prior art that integral formation of the strut and ring was not a viable or practical solution to the problem of improving the structural integrity of the valve structure and improving the blood flow across the valve. Increasing the size of the cross-sectional area of the wire, while leading increased structural integrity to the juncture between the wire strut and the valve ring, unavoidably increases the pressure gradient across the valve by offering a larger obstruction to the blood flow through the orifice defined by the ring.
As an alternative method, the machining of a disc-shape blank to form a valve ring integral with a strut in a single integral structure has met with similar difficulties, discussed below. Furthermore, the advantages to be gained by integral formation of the strut and ring have not been fully appreciated in the teachings of the prior art.